Problems with fogging have plagued the phographic industry from its inception. Fog may be formed from a deposit of silver or dye that is not directly related to the image-forming exposure; i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. % Fog can be used to measure the fog, which is (fogged silver/total silver coated).times.100.
It is known in the art that the appearance of fog can occur during many stages of preparation of the photographic element including silver halide precipitation, (spectro) chemical sensitization of the silver halide emulsion, melting and holding the liquid silver halide emulsion melts, and subsequent coating of silver halide emulsions. Such fog may occur due to trace amounts of metals such as iron, lead, tin, copper, nickel and the like from raw materials and manufacturing equipments.
Particularly, silver halide emulsions precipitated in the presence of ripeners such as thioethers or ammonia and/or reducing agents or environemts sensitive to reduction of silver ions such as high pH and/or low pAg usually suffer from high fog.
Several methods have been employed to minimize this appearance of fog as recently disclosed in U.S. Pat. No. 5,219,721 (1993) by R. Klaus et al and references therein. Klaus et al disclosed the use of organic dichalcogenides such as bis(p-acetamidophenyl)disulfide in sensitizing silver halide emulsions to combat fog. European Patent Application 0 566 074 A2 filed Apr. 14, 1993 by S. Kim et al disclosed use of such compounds in precipitation.
Research Disclosures 22534 (January, 1983) "Sensitized High Aspect Ratio Silver Halide Emulsions and Photographic Elements" and 308119 (December, 1989) "Photographic Silver Halide Emulsions, Preparation, Addenda, Processing and Systems" disclosed several methods which protect silver halide emulsions from fog and desensitization caused by metal contamination by incorporating addenda such as sulfocatechol-type compounds by Kenard et al U.S. Pat. No. 3,236,652; aldoximes by Carroll et al U.K. Patent 623,448; and meta- and poly-phosphates by Draisbach U.S. Pat. No. 2,239,284; and carboxyacids such as ethylenediamine tetraacetic acid by U.K. Patent 691,715. U.S. Pat. No. 3,300,312 by Willems et al disclosed use of sulfo-salicyclic acid type compounds to eliminate spot defects caused by metal contamination. U.S. Pat. No. 5,294,532--Ito et al discloses use of polyhydroxybenzene compounds for lowering fogging and improving pressure resistance.
Kenard et al U.S. Pat. No. 3,236,652 used sulfocatechol to eliminate desensitization and to prevent fogging caused by metal contamination by adding to the sensitized silver halide emulsions as a melt additive. It was not clear from the patent whether their emulsions contained optically sensitizing dyes or not. However, the use of optically (spectrally) sensitizing dyes during chemical sensitization was not readily known in the art until widespread use of tabular shaped silver halide emulsions.